Purified central nervous system myelin contains a small amount of a high molecular weight glycoprotein, referred to as the myelin-associated glycoprotein (MAG). This glycoprotein is selectively localized in the periaxonal portion of intact myelin sheaths. Peripheral myelin also contains a small amount of periaxonal MAG, but, in addition, the major structural protein of compact peripheral myelin (Po) is glycosylated. Since membrane glycoproteins are known to be involved in recognition and contact phenomena, these myelin-related glycoproteins are being studied with regard to their roles in glial-axonal interaction and myelin compaction. MAG of myelin undergoes a decrease in molecular weight during development which correlates well with myelin maturation. The chemical and immunological properties of mature and immature MAG are being determined. Since glycoproteins are known to be cell surface antigens and receptors for viruses, the possible involvement of MAG in the autoimmune or viral aspects of multiple sclerosis demyelinating diseases is being investigated.